Polyolefin/ionomer blend for improved properties in extruded foam products

ABSTRACT

Ionomer present in a polyethylene resin from about 1 to 40% by weight of the resin produces superior extruded foam sheet products that approach the pore size and resiliency of foams prepared from chemical blowing agents. The results can be achieved at normal extrusion rates and on standard extrusion foaming equipment. Tear strength is improved so that collapsible foam packaging products can be produced in which the foam body is cut through except for a surface thereof that remains intact to provide a hinge or joint about which the cut foam bodies can be pivoted.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.09/187,997, filed Nov. 6, 1998, U.S. Pat. No. 6,066,393 incorporatedherein by reference in its entirety, and claims the benefit of itsearlier filing date.

FIELD OF THE INVENTION

This invention relates to expanded cellular polyolefin products.

BACKGROUND OF THE INVENTION

One common method of preparing expanded cellular products is to mix amolten polymeric resin with a physical blowing agent in a zone of highpressure and to extrude the mixture into a zone of lower pressure wherethe blowing agent expands. Foams of low density and having a wide rangeof uses can be economically produced at high extrusion rates usingphysical blowing agents.

However, foams produced with physical blowing agents generally do nothave the fine cell structure, resiliency, and softness that can beachieved with chemical blowing agents, which tends to limit the marketfor extruded foams. High quality shoe soles, padded surfaces forexercise equipment, and protective padding can be made from foamsprepared with chemical blowing agents.

Foams that are prepared with chemical blowing agents normally areprepared in a two-stage process that is more troublesome and expensivethan extrusion foaming. Chemical blowing agents generally are notactivated until after the extrusion so that the extruded resin can becross linked. Chemical blowing agents release a high rate of gas,normally an inert gas, including nitrogen or carbon dioxide. In theabsence of cross linking, a chemical blowing agent generally wouldproduce a large number of open, coarse cells.

Foams can be cross linked by irradiation or by free-radical catalysts,including peroxides. In one method, a molten polymer resin is mixed withperoxide and a chemical blowing agent. The temperature is kept as low aspossible to avoid premature cross linking. Premature cross linking makesthe resin more difficult to extrude and increases the amount of heatgenerated. Extreme heat tends to produce foams that are unstable and aresubject to collapse. The resin is extruded and the extrudate is heatedto initiate cross linking and to activate the chemical blowing agent toproduce a foam.

Cheng-Shiang U.S. Pat. No. 4,738,810 describes a method of preparing afoam with a chemical blowing agent. The foam is prepared from linear lowdensity polyethylene. Excessive cross linking is said to be precluded bypremixing a chemical bowing agent, a cross linking agent, and otherpolymeric components before mixing with the linear low density resin.

Ionomer is said to be useful as one of the other polymeric components.Ionomers are copolymers having an ionizable comonomer. Ionomers arenormally prepared by copolymerization of ethylene with small amounts ofan unsaturated carboxylic acid, followed by ionization of the acid groupto yield a metal salt. The ionized groups act as meltable cross links.Ionomers have been used to improve the toughness and opacity to films,including the multilayer films that are used in vacuum skin packaging.Ionomers have also been used in resins for extrusion foaming undercertain circumstances.

For example, Watanabe et al. U.S. Pat. No. 4,102,829 discloses lowdensity extruded foams prepared from a mixture of from about 5 to 65%polyolefin and from about 35 to 95% ionomer. The foams are said to havea good balance of properties, including thermal resistance, and areindicated to be useful as an insulation covering on pipes for an airconditioner.

O'Brien et al. U.S. Pat. No. 4,091,136 discloses a synthetic cork-likematerial for use as a closure for liquid containers that is composed ofan extruded fine-celled polyolefin foam containing an ionomer. TheO'Brien et al. patent describes preparing a polyethylene foam rod withfrom 0.5 to 35 weight percent of DuPont Surlyn ionomer in the foamableresin mixture. The presence of ionomer is said to provide sufficientstructural strength to the polyolefin foam so that it can be handled inconventional corking equipment.

Cylindrical product profiles that characterize insulation covers for airconditioning pipes and synthetic cork generally result in relatively lowshear in the extrusion process, on the order of 10 sec⁻¹. Sheargenerates heat, which reduces melt strength and can be problematic,particularly at higher levels, resulting in unstable foams that tend tocollapse.

Extrusion foaming of sheet product profiles generally results in highershear in the extrusion process of about 100 sec⁻¹ or more. Shear can beseveral orders of magnitude greater for the production of sheet than forcylindrical product profiles.

High shear generation means that the heat generated by extrusion can beproblematic. The “processing window” of suitable operating parameters ofshear, melt temperature, and extrusion throughput for producing foamsheet products is relatively narrow compared to cylindrical productprofiles. The process of extrusion foaming of sheet products normallywill not tolerate cross linking in the resin, particularly at highershear rates. Cross linking can render a resin unprocessable,particularly at high shear. Accordingly, there is not believed to havebeen any disclosure or suggestion to incorporate ionomer into polyolefinresins for extrusion as sheet or plank.

It would be desirable if polyolefin foam sheet products could beprepared with the economies of the extrusion foaming process that couldbe competitive with foams prepared from chemical blowing agents.However, polyolefins are relatively low modulus polymers that normallydo not have the melt strength to form extruded foams of the fine cellstructure and resiliency that is achieved with cross linking andexpansion with chemical blowing agents.

Bambara et al. U.S. Pat. No. 5,876,813 discloses preparing an extrusionfoamed polyolefin foam structure that is said to have enhancedproperties by laminating a high density skin to a low density core. Thehigh density skin is said to improve the flexural stiffness and wearproperties of the foam structure. The structure is said to be useful incollapsible packaging systems because the structure can be die cut sothat the higher density skin can act as a hinge allowing the die cutpiece to be folded.

SUMMARY OF THE INVENTION

The invention relates to the use of ionomers in polyolefin resins forthe production of polyolefin foam sheet having improved properties, andin particular, to extrusion foamed polyolefin foam sheet that is usefulin collapsible packaging systems, whether in a laminated structure ornot. In laminated structures, the skin can be the same or nearly thesame density as the core. Whether in a laminated structure or not, theskin is of suitable tear strength and enhanced other properties to actas a hinge for die cut collapsible foams. It should be understood that,as used herein, the term sheet designates thin polyolefin extruded foamsof less than about ½-inch in thickness and also includes the thickerplank product profiles of up to about 3 inches or so.

It has been determined that polyolefin foam sheet can be produced by asingle stage extrusion foaming process having an acceptable processingwindow when ionomer is incorporated into the resin at particular levels.The foam can be made recyclable, which generally is not true of foamsprepared from chemical blowing agents. Foam sheet products havingimproved properties, including tear strength, thermoformability,cushioning, creep resistance, compression strength, hysteresis, and asofter touch, can be achieved at the high extrusion foaming throughputsthat are desired commercially. High throughput can be achieved onexisting single-stage extrusion equipment without having to make specialor difficult process adjustments.

The product of the invention, and, in particular, the skin that isformed on the surface of the product as a result of extrusion foaming,has tear strength sufficient for use as a hinge or joint between foambodies for collapsible packaging systems. The foam of the invention canbe cut through one surface and the core thereof to the other surfacewithout cutting through the other surface to create the hinge. The foamsheeting of the invention can be laminated to other structures, whichmay or may not be manufactured according to the invention, and used as ahinge.

At typical shear conditions for sheet products, the ionomer should bepresent in the polyolefin resin in an amount of from about 1 to 25% byweight, based on the polyolefin and ionomer components, for mostapplications. The extruded foam sheet product will have a similar amountof ionomer. Above about 25%, the processing window becomes narrow and itis more difficult to produce an acceptable product. However, atrelatively lower shear, it should be possible to use up to about 40% byweight ionomer in the resin. The temperature of the melt at the exit ofthe extruder should be maintained at less than or equal to about 238degrees Fahrenheit, and usually at about 230 degrees.

The density of the foam can range from about 20 to 150 kilograms percubic meter. For many packaging and thermoforming applications,including disposable trays for medical and dental instruments and thelike, the density is normally from about 20 to 50 kilograms per cubicmeter. The foam can be prepared with fine cells of from about 15 to 60cells per square inch. The foam can generally successfully resist animpact of from 200 to 425 pounds per square inch. The foam can beproduced as thin sheets of about one-half inch or less in thickness.Thicker plank foam products can be produced by extruding the foam insheets of one-half inch or greater thickness, up to about 2 inches, andby laminating two or more thin sheets to a desired thickness of up toabout 3 inches.

The resin mixture from which the foams are made can optionally include ametallocene polyolefin, which normally is a metallocene polyethylene,and which further strengthens the resin for expansion with a blowingagent. The metallocene generally is present in an amount of from about 5to 30% by weight of the resin. The density of the resin should normallybe maintained at or below about 0.930 g/cm³ for ease of single stepextrusion foaming.

Thus, the invention provides, among other benefits, an expanded cellularpolyethylene sheet product of low density having improvedcharacteristics that are competitive with foams prepared from chemicalblowing agents. The resin from which the product is made hasprocessability suitable for single stage expansion. The expanded productis recyclable and has improved tear strength, thermoformability, andpackaging characteristics without burdensome adjustments in processingconditions. Die cut collapsible foam packaging systems can be produced.While not wishing to be bound by theory, it is believed that the ionomerdevelops a reversible physical crosslinking within the foam thatprovides the improved strength properties. The bonds are thermallyreversible, which provides recyclability.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of foams prepared in accordance with the invention will now bedescribed with reference to the accompanying drawings and the severaltables that appear below in the Detailed Description of the Invention,in which drawings:

FIG. 1 is a evaluation of the cushioning ability, as a plot of peakacceleration for a particular static loading, of a 2-inch thicklaminated polyethylene plank of the invention compared to a standardlaminated 2-inch thick polyethylene foam product;

FIG. 2 is an evaluation of creep resistance, as a plot of the percentageof creep against time in hours, for a standard foam prepared in theabsence of ionomer;

FIG. 3 is an evaluation of creep resistance, as a plot of the percentageof creep against time in hours, for a foam prepared in accordance withthe invention, and illustrates the marked improvement in creepresistance due to the presence of ionomer in the resin from which thefoam is prepared;

FIGS. 4 and 5 are plots of extensional viscosity against strain forexamples of regular low density polyethylene sheet foam and for sheetfoams prepared with ionomer in the resin;

FIG. 6 is a perspective view of a die cut collapsible packaging systemfor a lap top computer (in shadow) prepared from an extruded foam sheetof the invention;

FIG. 7 is a top planar view of one portion of the system of FIG. 6showing the system folded (collapsed) for shipping;

FIG. 8 is a side planar view of the subject of FIG. 7;

FIG. 9 is a top planar view of one portion of the system of FIG. 6 shownopened for use as in FIG. 6;

FIG. 10 is a side planar view of the subject of FIG. 9; and.

FIG. 11 is a perspective view similar to that of FIG. 6, but showing alaminated foam structure prepared in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

Various processes and equipment for extrusion foaming of thermoplasticresins have been used for many years. Generally, solid pellets ofthermoplastic resin are fed through a hopper to a melting zone in whichthe resin is melted, or plasticized, to form a flowable thermoplasticmass. The plasticized thermoplastic mass generally is then metered to amixing zone where the thermoplastic mass is thoroughly mixed with ablowing agent under pressure for subsequent cooling and expansion of theresin to form a foam. Blowing agent typically is injected between themetering and the mixing zones. The blowing agent can be injected througha single port or multiple ports. Multiple ports can be used fordifferent blowing agent components or the components can enter through asingle port. The mixture of thermoplastic resin and blowing agent isthen forced through a die, which imparts a shape to the thermoplasticmass, into a zone of lower pressure, such as atmospheric pressure. Theblowing agent expands to form the cells of the foam and thethermoplastic foam is cooled.

Typical of much of the equipment used for extrusion of thermoplasticfoams, the thermoplastic pellets are conveyed from a hopper through themelt zone and the mixing and cooling zones, and is then extruded throughthe die by a screw type apparatus. Single screw extruders are common,although double screw extruders sometimes are used for greater mixing.Double screw extruders can be either twin screw, in which the resinpasses through two screws in parallel, or tandem screws, in which theresin passes through two screws in series.

When a blowing agent is injected into the mixing zone of the screwextruder, the blowing agent initially forms a dispersion of insolublebubbles within the plasticized thermoplastic mass. These bubbleseventually dissolve in the thermoplastic mass as the mixing continuesand the pressure increases down the length of the extruder. The extrudershould have a length to diameter ratio of at least 30:1 and a sufficientlength of mixing zone to ensure that proper mixing occurs.

Thermoplastic resins contemplated for use in the practice of theinvention claimed herein include the polyolefin resins. Polyolefinresins may be defined as polymers derived from unsaturated hydrocarbonscontaining the ethylene or diene functional groups. Polyolefin resinsmay include virtually all of the addition polymers, however, the termpolyolefin typically is used for polymers of ethylene, the alkylderivatives of ethylene (the alpha-olefins), and the dienes. Among themore commercially important polyolefins are polyethylene, polypropylene,polybutene, and their copolymers. Polyethylene resins are particularlyuseful in the practice of the invention claimed herein.

Polyethylene is a whitish, translucent polymer of moderate strength andhigh toughness. Polyethylene is available in forms ranging incrystallinity from 35 to 95 percent. Polyethylene is available in low,medium, and high density polymer forms. For the low density material,the softening temperature is about 105° C. to 115° C. For the highdensity material the softening temperature is some 25° C. to 40° C.higher, or from about 130° C. to 140° C. Low, medium, and high densitypolyethylenes are suitable for extrusion foaming, including mixturesthereof.

The thermoplastic resin should be maintained at a temperature within arange above the melting point of the polymer that is sufficiently highso that the polymer has sufficient fluidity for mixing with blowingagent. This range normally will be from about 20° C. to 100° C. abovethe melting point of the resin. The melting zone can be maintained at asomewhat lower temperature due to the heat that is generated by frictionas the plasticized resin flows through the extruder.

After mixing, the temperature of the mixture of resin and blowing agentshould be lowered closer to the melting point of the mixture so that thepolymer maintains its structure upon foaming, but not so much thatcomplete expansion is hindered. The blowing agent has a plasticizingeffect on the resin reducing its viscosity, or resistance to flow, andso the melting point of the mixture of resin and blowing agent normallyis below that of the resin alone. The expansion temperature, which isabove the melting point of the mixture, is empirically determined anddepends upon the composition of the resin, the length of the screw,whether single or double screws are used, on the specific resin, uponthe amount of blowing agent, and the specific blowing agent blend. For alow density polyethylene, the expansion temperature will generally be inthe range of from about 85° C. to 120° C.

The blowing agent contemplated for use in practicing the inventionshould preferably comprise those that are acceptable in commercialpractice. Various volatile organic compounds (“VOCs”) are useful blowingagents, including the light aliphatic hydrocarbons ethane, propane,butane, and others, used alone or in mixtures, including mixtures withhalogenated hydrocarbons and/or various inert gases, including nitrogenand carbon dioxide. Halogenated hydrocarbons, including clorofluorinatedhydrocarbons (“CFCs”), and mixtures of halogenated hydrocarbons can beused in connection with practice of the invention. However, governmentalregulation is phasing out use of many halogenated hydrocarbons becausethese compounds may be responsible for damage to the earth's ozonelayer.

An ionomer should be mixed with the polyolefin resin to form ahomogeneous resin prior to mixing with blowing agent and expansion.Ionomers are copolymers of ethylene and a vinyl monomer with an acidgroup such as methyacrylic acid. The ionomers are cross-linked polymersin which the linkages are ionic and covalent bonds. There are positivelyand negatively charged groups that are not associated with each other,and this polar character makes these resins unique. One example of anionomer that is useful in the practice of the invention is sold underthe mark Surlyn, which is available from DuPont. Several Surlynformulations are available.

The benefits of the invention can be achieved with the use of relativelysmall amounts of ionomer in the polyolefin resin. The ionomer shouldtypically be present in admixture with the polyolefin in an amount offrom about 1 to 25 percent by weight of the admixture. Generallyspeaking, benefits of the invention can be achieved with from about 1 to10 percent ionomer by weight of the resin. Up to about 40 percentionomer can be used if the shear that is developed at the exit of theextruder, where the greatest shear is developed, is not too great.

Shear tends to promote heat and the formation of open cells. Crosslinking leads to shear development, and so it is desirable that no crosslinking occur prior to extrusion. Shear rate for producing polyolefinfoam sheet can be as low as about 80 to 100 sec⁻¹ and normally are lessthan 1,000 sec⁻¹. However, shear rates of from about 1,000 to 2,000sec⁻¹ are somewhat more common.

While not wishing to be bound by theory, it is believed that theionomer, at the relatively low temperatures at which polyolefin resinsare extrusion foamed, forms physical, ionic bonds in the resin. Ionicbonds are to be distinguished from the covalent, chemical bonds formedby a typical cross linking agent. The ionic cross links continue to formas the foam cools after extrusion, further strengthening the foam. Theionic bonds are thermally reversible, which renders the foam recyclable.

Polyolefins are generally of low modulus, which is to say that thepolymers have relatively low stiffness, tensile strength, andcompression resistance. Ionomer cross linking of the extruded resinimproves the characteristics of the foams, including stiffness, tensilestrength, and compression resistance.

Metallocene-catalyzed ethylene/alpha-olefin copolymers (“metallocenepolyethylenes”) are also useful in admixture with the resin of theinvention and can further improve the strength of the resin forextrusion foaming. Metallocene catalysts are organometallic coordinationcompounds obtained as a cyclopentadienyl derivative of a transitionmetal or metal halide. The metallocene catalyst produces homogeneousethylene/alpha-olefins, unlike the more traditional Ziegler-Nattacatalysts that produce heterogeneous ethylene/alpha-olefins. Whenpresent, the metallocene polyethylene should be present in an amount offrom about 5 to 30 percent by weight of the resin, based on thepolyethylene, ionomer, and metallocene polyethylene in the resin. Thedensity of the resin should be kept at or below 0.930 g/cm³.

Table 1, below, compares the thermoformability of foams prepared inaccordance with the invention with that of a standard low densitypolyethylene foam. The foams have similar cell count. Thermoformabilityis visually observed. The presence of a small amount of ionomer, 5% byweight of the resin, and a metallocene linear low density polyethyleneat about 24% by weight in a low density polyethylene resin produced afoam having very good thermoformability properties as compared to astandard polyethylene foam in the absence of ionomer and metallocenepolyethylene. Metallocene linear low density polyethylene increases themelt strength of the resin. However, it should be noted that 1 to 2% byweight ionomer should be sufficient in many resins in the absence ofmetallocene to produce foams having improved characteristics.

TABLE 1 mLLDPE LDPE Metallocene Ionomer Cell Counts Kg/hr Kg/hr Kg/hr#/cm Thermoform 1 136.4 0 0 19 N/A 2 102.3 32.1 0 20 Good 3 95.5 32.16.8 19 Very Good

Extruded foams for thermoformed products should have severalcharacteristics. Thin sheets for thermoformed trays should be finecelled and of low density, usually from about 20 to 50 kg/m³. The foamshould have a pleasing hand that is not unpleasant to touch.

One important characteristic of foams suitable for thermoforming isthermal stability, which is the ability to withstand the elevatedtemperatures of the thermoforming process over an extended period oftime. Thermal stability is not the same as resistance to thermaldeformation, which is also referred to as thermal resistance. Thermalresistance generally refers to the ability of a foam to withstanddeformation at a particular elevated temperature. Thermal resistance isusually associated with foams that bare used in insulating capacities orare otherwise subjected to elevated temperatures in use.

Table 2, below, compares the compression strength of a standard lowdensity polyethylene foam with that of a foam containing 10 percent byweight ionomer. These 2-in. (5.1 cm) sheets are soft, resilient boxfoams useful for packaging applications. It can be seen that at each ofthe various compression levels, the compression strength of the foamcontaining ionomer is improved as compared to a foam without theionomer. The effect is more pronounced at higher compression levels, andthe ionomer foam generally shows good cushioning performance at higherloads.

TABLE 2 Compression Strength (psi) Material 25% 50% 80% Standard Foam5.28 13.33 51.52 Foam with 10% Ionomer 5.54 14.05 54.09

As shown in Table 3, below, the properties of peak stress, hysteresis,and the difference between loading and unloading energy (resiliency) areall improved with the addition of 10 percent ionomer to polyethylenefoam. The lower difference between loading and unloading energy for theionomer foam indicates that the ionomer foam recovers more fully aftercompression. In general, the ionomer foams show less cell wall damage,less hysteresis, and better resiliency.

TABLE 3 Loading Unloading Energy Energy Hysteresis Peak Stress (in xlbs) (in x lbs) (%) Standard Foam 42.52 psi 317.06 245.20 22.66 Foamwith 10% 48.68 psi 380.23 320.92 15.60 Ionomer

Table 4, below, compares the properties of a standard polyethylene foamsheet, Example 1, to the properties of a foam sheet of 1.3 cm thicknessthat was made on a 16.8 cm counter-rotating twin screw at 380 Kg/hr(Example 2). Example 1 includes talc as a nucleating control agent.Example 2 was prepared with Hydrocerol CF20, which is a brand ofnucleating control agent available from B.I. Chemical. The two differentnucleating control agents are not believed to be responsible for thedifferences between the two foams as shown in the table. Resistance tocompression was determined at 25% and at 50% compression, and at eachcompression, the ionomer foam showed greater resistance to compression.Hysteresis was determined as the inverse energy ratio of loading to 80%compression followed by decompression to the original volume. It shouldbe noted that hysteresis was reduced by the addition of ionomer to thepolyethylene foam and that the ionomer foam showed less impact damage.

TABLE 4 1 2 Ionomer 0 10.5% Density, Kg/m³ 28.3 25.6 Cell Counts, #/inch19 26 Compression, psi 25% 5.3 5.5 50% 13.3 14.1 Hysteresis % 22.7 15.6Drop impact difference between 1st and 5th drop, G′ Load 0.25 psi 4 2 0.5 psi 4 3  1.0 psi 6 3  1.5 psi 8 4  2.5 psi 22 14

It can be seen in Table 4 that the example of foam having ionomertherein showed less impact damage, as indicated by the differencebetween the first and last impact drops, than examples without ionomer.G′ is defined as an absorption factor and is an indication of theability of a foam to absorb an impact. The lower the value of G′, themore energy is absorbed by the foam. The drop impact test is performedby dropping a weight (static load) on the foam sample from a fixedheight of 24 inches. Each weight is dropped five times, and the amountof impact energy absorbed by the foam from each drop is determined. Thedifference in impact energy absorbed between the first and fifth dropsfor a particular weight is reported below in Table 5 for each of fivedifferent weights. The foam with 10.5% by weight ionomer performedbetter than foam in the absence of ionomer. There is less differencebetween the first and fifth drops under the same weight for the ionomerfoam, which indicates less damage to the foam after repeated drops.

Table 5, below compares the ability to withstand impact and otherproperties of four foams of 5.1 cm thickness in a manner similar toTable 4.

TABLE 5 1 2 3 4 talc, Kg/hr — 0 0 0 Nucleator, CF20, kg/hr 0 4 4 1.2Ionomer 0 0 10% 10% Density Kg/m³ 19.4 25 25.1 21.8 Cell Count #/inch 1625 24 17 1st and 5th drop impact difference Load 0.25 psi 5.8 10 3.3 0.4 0.5 psi 3.7 3.6 0.8 3.6  1.0 psi 4.6 2.8 3.1 3.5  1.5 psi 5.3 7.4 4.05.1  2.5 psi 25.8 16.7 12.8 16.2

Table 6, below, compares 2-inch samples (5.1 cm) of foam, one preparedwith ionomer and one without, at 5 different static loads (in units ofpsi). Foam having 10% ionomer showed better impact energy absorption athigher static loads than did the standard foam, which is an indicationof superior cushioning properties.

TABLE 6 Loading (PSI) Density 0.25 0.50 1.00 1.50 2.50 Standard 2″ 27.2kg/m³ 53 51 61 68 123 Foam Foam with 2″ 26.4 kg/m³ 53 50 55 64 113 10%Ionomer

FIG. 1 is a plot of peak accelerations (G′) against static loading inlbs/in² (psi) for 2-inch thick laminated polyethylene plank of theinvention compared to a standard laminated 2-inch thick polyethylenefoam product. The plot shows clearly that the foam prepared with ionomerhas better impact resistance across a range of static loads, which is anindication of improved cushioning performance for the ionomer containingfoam.

FIG. 2 is an evaluation of creep resistance. The percentage of creep isplotted against time and hours for a standard foam prepared in theabsence of ionomer. The same curve plotted for a polyethylene foamcontaining 10% ionomer is shown in FIG. 3. A comparison of these curvesshows that the percentage creep is markedly reduced for the polyethylenefoam containing ionomer.

Table 7, below, shows several examples of foams containing varyingamounts of ionomer, from 0 to 36.4 kg/hr at an overall rate of 173kg/hr. The ionomer is DuPont Surlyn 8273. The ionomer containing foamsshow improvement over that without with respect to impact resistance atconstant force and machine and cross direction tensile and elongation.

TABLE 7 1 2 3 4 5 6 7 Nucleator 4.5 4.5 7.7 7.7 7.7 4.5 4.5 CF20, kg/hrIonomer, kg/hr 0 36.4 36.4 18.2 18.2 <8 0 Density, Kg/m³ 55 91.5 88.387.2 84.2 87 82.9 Cell Counts, — — — — 45 50 55 #/inch Impact, psi 113412 313 247 202 217 177 Tensile, psi MD 76 217 181 147 109 146 125 CD 56106 98 94 96 99 85 Elongation, % MD 121 125 125 106 120 116 137 CD 75107 101 95 93 89 85

FIGS. 4 and 5 are plots of extensional viscosity against strain forsamples of regular low density polyethylene sheet foams prepared withionomer in the resin in accordance with the invention. Extensionalviscosity is in the units of Pascal x seconds. Extensional viscosity isa measure of the ability of the material to resist extensional stress.Extensional viscosity is determined by stretching to extend a molten rodof resin to the breaking point, and measuring the resistance. Theseplots show clearly that the presence of ionomer in a low densitypolyethylene resin produces a foam that has superior resistance tostress as compared to a low density polyethylene foam in the absence ofionomer.

The samples for FIGS. 4 and 5 were prepared with various DuPont Surlynionomers. DuPont Surlyn 1652 has a meltflow index of 5.2 g/10 min with azinc ion. DuPont Surlyn 1706 has a meltflow index of 0.7 g/10 min with azinc ion. Surlyn 8920 resin has a meltflow index of 1.0 with a sodiumion.

Tables 8 and 9 below, show data obtained from heat shrinkage tests. Thisdata shows that foams with ionomer have improved thermoformabilityproperties. The ionomer foams show smaller dimensional changes with theapplication of heat over time, which is a reflection of thermalstability. The data in the tables was taken from a square sample of foamthat measured 30.4 cm on each side. In Table 8, the foam was kept in anoven at 66.7° centigrade for 3 hours and that at 20° centigrade for 1hour. In Table 9, the foams were kept for 3 hours in an oven at 80°centigrade and for 1 hour at 20° centigrade. Several of the sampleswithout ionomer curled after heating in the oven for 3 hours.

TABLE 8 Foam Gauge Metallocene Ionomer Dimension Change 6.35 mm* 25% 0MD: −2.4% CMD: +0.8% 6.35 mm* 25% 0 MD: −3.7% CMD: +1.3% 6.35 mm 25% 5%MD: +0.5% CMD: −0.8% 3.17 mm 25% 5% MD: −0.3% CMD: −0.8% 3.17 mm  0 0MD: −2.4% CMD: +1.3%

TABLE 9 Foam Gauge Metallocene Ionomer Dimension Change 3.17 mm  0 0 MD:−1.3% CMD: +0.3% 3.17 mm 25% 0 MD: −2.1% CMD: +0.5% 3.17 mm 25% 5% MD:−0.8% CMD: +0.5% 1.58 mm 25% 5% MD: −1.1% CMD: +0.3% 6.35 mm* 25% 0 MD:−4.5% CMD: +1.3% 6.35 mm 25% 5% MD: −2.4% CMD: +0.5% *Sample curledafter oven heating for three hours.

Table 10, below, compares the tear strength and other physicalproperties of an ionomer foam sheet of the invention, Example 2, with afoam sheet produced in the absence of ionomer, Example 1. The tearstrength, tensile strength, and percent elongation were all increased inboth the machine and tear directions in the foam sheets containingionomer. The examples were prepared using a 150 mm counter-rotating twinscrew extruder. The base resin was a low density polyethylene resinhaving a melt index of 3.5. The ionomer was DuPont Surlyn 1652. Themetallocene was DuPont Affinity 1845. Product performances are as setforth below:

TABLE 10 Sample 1 2 LDPE, wt % 100 70 Metallocene, wt % 0 25 Ionomer, wt% 0 5 Foam Thickness, mm 3.8 3.3 Density, Kg/m³ 28.2 31.8 Tear Strength,lb/in MD 10.95 15.54 TD 18.80 21.15 Tensile Strength, lb/in² MD 66.6776.74 TD 31.33 45.91 Elongation, % MD 23.08 24.08 TD 7.69 17.95

The increase in tear strength as shown in Table 10 that is achieved bythe practice of the invention is sufficient for preparing collapsiblepolyethylene foam sheet packaging materials in which the sheet is cutthrough one skin and the core, leaving the skin on the opposite sideintact. The sheet can be pivoted about the intact skin as a hinge. Thecut sheet forms a leaf connected to the foam sheet by the intact skin,which acts as a joint or hinge. Multiple cut portions can allow thepackaging material to be collapsed into a compact condition and unfoldedfor packaging, if desired. It should also be recognized that a portionof the core wider than a slit can be removed to provide greaterflexibility in the hinge and to allow the foam leaves to be folded awayfrom the hinge side of the foam sheet product. Density of the hingedproducts of the invention can vary from 15 to 150 kg/m³, but typicallyare from 20 to 50 kg/m³.

The invention thus provides an extrusion foamed sheet that convenientlycan be cut through to the skin and has sufficient tear strength forpackaging applications. It should also be recognized that the foam sheetproduct can be prepared from multiple foam sheets that are laminatedtogether. At least one foam sheet is laminated to a core and is preparedin accordance with the invention to provide a hinge about which a cutleaf of the core can be pivoted. At least the hinge portion of thelaminate should incorporate ionomer as set forth above. Thickness of thefoam sheet product will vary. Commonly, the core will be from aboutone-half to three inches thick and the surfaces laminated to the corewill typically be less than about a half inch thick, although otherdimensions are contemplated depending on individual use. In a laminatedstructure, the outer laminated sheet can be the same or nearly the samedensity as the core sheet, if desired, or can be of lessor or greaterdensity, usually greater rather than lessor.

An embodiment of a collapsible packaging system of the invention isshown in perspective in FIG. 6. A collapsible packaging system is showngenerally at 20 and includes two end caps 22 and 24 that are placed onopposite ends of a laptop computer, which is shown in shadow. The endcaps 22 and 24 are extruded foam sheets prepared in accordance with theinvention. The extruded foam sheets typically are of about 2 inches inthickness, although the thickness can be varied depending upon theparticular application desired. It should be noted that end caps 22 and24 can be laminated products that are prepared from several sheets ofthe same density to reach a 2 inch thickness or can be produced as asingle extruded sheet or plank. Typically, these sheets will be preparedfrom a polyethylene resin that includes ionomer in accordance with theinvention. At least the outer most sheet in a laminated structure shouldinclude ionomer in accordance with the invention.

End caps 22 and 24 each include die cut leaves 26 and 28, respectively,that are shown folded outwardly of the packaging system so as to formsupport structures that complete the suspension for the packagedproduct, which in this embodiment is a laptop computer. The leaves 26,28 have hinge portions 30 and 32, respectively, (FIG. 7) about which theleaves are pivoted as they are rotated to extend outwardly from the endcaps 22, 24.

FIG. 7 shows a planar top view of end cap 24 of FIG. 6. In theembodiment illustrated in FIG. 7, the leaves 26 are folded into positionin end cap 24 in a collapsed configuration suitable for shipping to acustomer for use as a packaging system. The end cap is produced as asingle extruded sheet and has been die cut to produce the shapes of theleaves 26 and to leave a portion of the skin 34 on the outer surface ofend cap 24 intact for use as a hinge 30 about which the leaf 26 isrotated. An end view of the collapsed structure of FIG. 7 is shown inFIG. 8.

Die cutting of a hinge on a polyethylene foam sheet as illustrated inFIG. 6 is easily accomplished by precise cutting of the foam bodythrough the foam body to a point adjacent the skin on one side thereof,without cutting through the skin. This method of cutting is sometimesreferred to as “kiss cutting.” The die plate against which the foam isplaced when cutting should contain areas that are recessed where thehinge location is desired. The die press is otherwise set up in a mannersimilar to that for cutting a shape completely through both surfaces andthe core. As can be seen in FIGS. 6 and 7, the die has cut a shapecompletely through the foam body along solid lines 36 and the foam bodyhas been completely cut through on all sides of portions 38, which havebeen removed from the foam body for ease in folding the skin at the coreand opening the packaging system. The hinge 30 is formed from uncutmaterial that remains after the die plate has penetrated the totalthickness of the original material in the surrounding areas, but has cutthrough only the opposite surface and the core in the region of thehinge. It should be recognized that the cut defining the hinge does nothave to extend all the way through to the uncut skin portion of thehinge, but that the hinge can include a portion of the core, if desired.

FIGS. 9 and 10 show top and side planar views of end cap 24 of FIG. 6opened for use as shown in perspective in FIG. 6. The die cut portionscan be clearly seen where the leaves are folded outwardly from the endcap.

FIG. 11 is a perspective view of an embodiment of the invention in whicha laminated structure has been used for the collapsible packagingsystem. Analogous portions of the collapsible packaging system to thoseportions of FIG. 6 are shown by use of numerals indicated to be primes.The laminated structure is shown generally at 20′. End caps 22′ and 24′are shown mounted on the ends of a laptop computer, which is shown inshadow and ready for placement in a shipping box or other container. Endcaps 22′ and 24′ in this embodiment are laminated structures comprisingcore foams 23′ and 25′, respectively, laminated to thinner skin foams35′ and 34′ respectively. The core foam can be formed of any suitablelow density polyolefin resin, and does not necessarily need to beprepared with ionomer in the polyolefin resin composition. However, corefoams 23′ and 25′ are part of a laminated structure that includesthinner foams 35′ and 34′, respectively, that form the outer surface, orskin, of the laminated structure. Skins 35′, 34′ can be laminated tocore foams 23′, 25′, respectively, by any suitable lamination means.Skins 34′ and 35′ are prepared with a polyolefin resin that includesionomer in accordance with the invention and is a single stage extrudedfoam.

The laminated foam body is die cut through the core structures 23′, 25′and through the outer skin 35′, 34′, respectively, except for thoseportions of the skin or of the skin and a portion of the core that forma hinge. Leaves 26′ and 28′ outwardly of the structures on hinges asdescribed with respect to FIG. 6. Hinge 30′ is shown on the end cap 24′.The hinge on end cap 22′ is not illustrated.

The skins 34′ and 35′ have sufficient tear strength and other propertiesto enable their use as hinges about which leaves can be folded outwardlyof the end cap body. For laminated structures as shown in FIG. 11, thecore density typically is from about 20 to 40 kg/m³. The skin istypically of density from about 29 to 40 kg/m³. It should be noted thatthe core and skin structures can be of the same or similar density.However, it should also be recognized that, if desired, these structurescan be of different density. In this case, the skin foam typically willbe of somewhat greater density than the core foam.

Particular embodiments of the invention have been described in detail inthe tables and drawings and specific terms have been used. It should beunderstood in a generic and a descriptive sense and not for purposes oflimitation, the scope of the invention as defined by the claims.

What is claimed is:
 1. An extruded foam sheet product having first andsecond skin surfaces defining a core thickness dimension therebetweenand wherein said product is, cut through said first surface and saidcore without cutting through said second surface thereby defining atleast one leaf connected to said core by a joint defined by said secondsurface about which said leaf pivots, said skin surfaces and said corecomprising a homogeneous admixture of about 70% by weight low densitypolyethylene, 5% by weight ionomer, and 25% by weightmetallocene-catalyzed ethylene/alpha-olefin copolymer.
 2. The foam sheetof claim 1 wherein said product has from about 15 to 60 cells per squareinch.
 3. Extruded foam sheet product having first and second surfacesdefining a core thickness dimension therebetween, wherein said productis cut through said first surface and said core without cutting throughsaid second surface thereby defining at least one leaf connected to saidcore by a joint defined by said second surface about which said leafpivots, said second surface comprising a homogeneous admixture ofpolyolefin and ionomer in an amount of from about 1 to 40% by weight ofsaid admixture, and wherein said core and said second surface have aboutthe same density.
 4. The foam sheet product of claim 3 wherein saidproduct is a single extruded sheet, said first and second surfaces areskin surfaces formed on said core as a result of extrusion foaming ofsaid core, and said core and said first surface have the samecomposition as said second surface.
 5. The foam sheet product of claim 3wherein said product has a density of from about 15 to 150 kg/m³.
 6. Thefoam sheet product of claim 3 wherein said second surface and said coreare separate foam sheets that have been laminated together to form aunitary structure.
 7. The foam sheet product of claim 6 wherein saidcore is from ½ to 3 inches thick and has a density of from 20 to 50kg/m³ and wherein said surface is a foam sheet of {fraction (1/2 )} inchor less thick and has a density greater than said core.
 8. The foamsheet product of claim 6 wherein said first surface is a skin surface ofsaid core formed on said core as a result of extrusion.
 9. The foamsheet product of claim 3 wherein said core and said first surface alsocomprise a homogeneous admixture of polyolefin and ionomer in an amountof from about 1 to 40% by weight of said admixture.
 10. The foam sheetproduct of claim 3 wherein said first surface and said core are separatefoam sheets that have been laminated together.
 11. The foam sheetproduct of claim 3 wherein said polyolefin is low density polyethyleneand said admixture further comprises metallocene polyethylene present inan amount of from about 5 to 30% by weight of said admixture.
 12. Thefoam sheet product of claim 3 wherein said polyolefin is low densitypolyethylene present in said admixture in an amount of from about 45 to93% by weight of said sheet.
 13. The foam sheet product of claim 3wherein said product is recyclable.
 14. The foam sheet product of claim3, wherein ionomer is present in an amount of from about 1 to 10% byweight.
 15. Extruded foam shoot product having first and second surfacesdefining a core thickness dimension therebetween and wherein saidproduct is cut through said first surface and said core without cuttingthrough said second surface thereby defining at least one leaf connectedto said core by a joint defined by said second surface about which saidleaf pivots, said second surface comprising a homogeneous admixture oflow density polyethylene, ionomer in an amount or from about 1 to 40% byweight of said admixture, and metallocene polyethylene present in anamount of from about 5 to 30% by weight of said admixture.
 16. The foamsheet product of claim 15 wherein said product is a single extrudedsheet, said first and second surfaces are skin surfaces formed on saidcore as a result of extrusion foaming of said core, and said core andsaid first surface have the same composition as said second surface. 17.The foam sheet product of claim 15 wherein said product has a density offrom about 15 to 150 kg/m³.
 18. The foam sheet product of claim 15wherein said second surface and said core are separate foam sheets thathave been laminated together to form a unitary structure.
 19. The foamsheet product of claim 18 wherein said core is from ½ to 3 inches thickand has a density of from 20 to 50 kg/m³ and wherein said surface is afoam sheet of ½ inch or less thick and has a density greater than saidcore.
 20. The foam sheet product of claim 18 wherein said first surfaceis a skin surface of said core formed on said core as a result ofextrusion.
 21. The foam sheet product of claim 15 wherein said core andsaid second surface have about the same density.
 22. The foam sheetproduct of claim 15 wherein said core and said first surface alsocomprise a homogeneous admixture of polyolefin and ionomer in an amountof from about 1 to 40% by weight of said admixture.
 23. The foam sheetproduct of claim 15 wherein said first surface and said core areseparate foam sheets that have been laminated together.
 24. The foamsheet product of claim 15 wherein said polyolefin is low densitypolyethylene present in said admixture in an amount of from about 45 to93% by weight of said sheet.
 25. The foam sheet product of claim 15wherein said product is recyclable.
 26. The foam sheet product of claim15 wherein ionomer is present in an amount of from about 1 to 10% byweight.